Add contributing instructions.

CONTRIBUTING.rst

@@ -1,4 +1,72 @@
-Contributing to ``pycodestyle``
-===============================
+Contributing to pycodestyle
+===========================
 
-Please see the `developer notes <https://pycodestyle.readthedocs.io/en/latest/developer.html>`_
+When contributing to pycodestyle, please observe our `Code of Conduct`_.
+
+Step 1: Forking pycodestyle for editing
+---------------------------------------
+
+Fork the pycodestyle repository on GitHub. This will add
+pycodestyle to your GitHub account. You will push your changes to your
+fork and then make pull requests into the official pycodestyle repository.
+
+GitHub has an excellent `guide`_ that has screenshots on how to do this.
+
+Next, clone your fork of the pycodestyle repository to your system for
+editing::
+
+    $ git clone https://www.github.com/<your_username>/pycodestyle
+
+Now you have a copy of the pycodestyle codebase that is almost ready for
+edits.  Next we will setup `virtualenv`_ which will help create an isolated
+environment to manage dependancies.
+
+
+Step 2: Use virtualenv when developing
+--------------------------------------
+
+`virtualenv`_ is a tool to create isolated python environments.
+First, install virtualenv with::
+
+    $ pip install virtualenv
+
+Next, ``cd`` to the pycodestyle repository that you cloned earlier and
+create, then activate a virtualenv::
+
+    $ cd pycodestyle
+    $ virtualenv pycodestyle-venv
+    $ source pycodestyle-venv/bin/activate
+
+Now you can install the pycodestyle requirements::
+
+    $ pip install -r dev-requirements.txt
+
+To deactivate the virtualenv you can type::
+
+    $ deactivate
+
+For more information see `virtualenv`_'s documentation.
+
+
+Step 3: Run tests
+-----------------
+
+Before creating a pull request you should run the tests to make sure that the
+changes that have been made haven't caused any regressions in functionality.
+To run the tests, the core developer team and Travis-CI use `tox`_::
+
+    $ pip install -r dev-requirements.txt
+    $ tox
+
+All the tests should pass for all available interpreters, with the summary of::
+
+    congratulations :)
+
+At this point you can create a pull request back to the official pycodestyles
+repository for review! For more information on how to make a pull request,
+GitHub has an excellent `guide`_.
+
+.. _virtualenv: http://docs.python-guide.org/en/latest/dev/virtualenvs/
+.. _guide: https://guides.github.com/activities/forking/
+.. _tox: https://tox.readthedocs.io/en/latest/
+.. _Code of Conduct: http://meta.pycqa.org/en/latest/code-of-conduct.html